1. Field of the Invention
The present invention relates to a reflection type photoelectric switching apparatus comprising a photoelectric switching unit having a light projecting portion and a photo sensitive portion and a reflex reflector arranged at a position which is away from the photoelectric switching unit by a predetermined distance, wherein the photoelectric switching unit and reflex reflector are arranged in a manner such that a projection light from the light projecting portion is projected onto the reflex reflector and the reflected light from the reflex reflector is detected by the photo sensitive portion.
2. Prior Art Statement
In the above photoelectric switching apparatus of the reflex reflecting type, since an amount of reflected light which is detected by the photo sensitive portion changes when an object passes through the region between the photoelectric switching unit and the reflex reflector, the object is detected on the basis of such a change. However, when the object has the mirror surface (metal glossy surface or the like), almost all of the projection light is reflected by the object and its reflection is the mirror surface reflection in which an incident angle and a reflection angle are equal. Therefore, almost all of the reflected light is detected or is not detected by the photo sensitive portion in dependence on the position of the object (distance from the object to the photoelectric switching unit), so that the reliability is low.
To solve such a drawback, as shown in JP-A-59-119628, there has been proposed a reflex reflection type photoelectric switching apparatus in which the lights of two different wavelength bands, for instance, the red light and infrared light are irradiated onto a reflex reflector by using two light projecting devices, the reflex reflector is constructed so as to reflect only the infrared light and to absorb the lights of wavelength bands other than the infrared light, and by mutually comparing the levels of light detection signals of the two kinds of lights, the reflex reflector and object are distinguished.
However, such a reflex reflection type photoelectric switching apparatus has a drawback such that an object having the print color such as blue, purple, or the like cannot be distinguished from the reflex reflector. That is, as shown in FIG. 6, an object having the color of blue, purple, or the like absorbs the red light and reflects the infrared light having a long wavelength and has the same spectrum characteristic as that of the reflex reflector. Therefore, there is a drawback such that the reflex reflector locating at a remote distance and the blue object existing at a near distance cannot be distinguished and the object cannot be detected.
On the other hand, the foregoing reflection type photoelectric switch has a drawback such that two light projecting portions are necessary and the number of parts of the optical system increases. Further, since the optical axes of the two light projecting portions are slightly deviated, there is a problem such that if the object to be detected shuts out only the light beam of either one of the optical axes, there is a fear such that a detection signal is erroneously output.